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I am new on TeXmacs, and I haven't found how to do big left brackets on the 
left of equations, for stating different cases.
The LaTeX equivalent would be, for the absolute value function for example:
\begin{equation}
abs(x)=
 \begin{cases}
  -x & x<0
  x & x>=0
 \end{cases}
end{equation}

If someone can help ^^